Check the carbuerator setting. If the carb is not properly adjusted (usually turned open slightly more for high altitude. ) the mower will stall, smoke or generally run poorly
You may need to adjust the carburetor. This is due to lower than normal oxygen levels at higher altitudes (typically, between 3,000 to 4,000 feet above sea level).
Problems with altitude usually have to do with the jetting of the carburator. If jetting isn't corrected for altitude it usually runs poor because it is too rich.
could experience a hard time idling along with trouble mainting higher rpm's. since there is less oxygen at higher altitudes you need to adjust the carb to less fuel.